James Dolezal argues that if God is immutable in ethics, then this requires a metaphysical explanation:
Divine immutability enjoys more explicit biblical affirmation than doctrines such as divine aseity and infinity.44 Many of the supporting passages tend to focus on the constancy and faithfulness of God to do what he has promised to do, that is, upon his ethical immutability. Nevertheless, even ethical immutability requires an ontological explanation rooted in the very being and essence of God.
Dolezal, James E.. God without Parts: Divine Simplicity and the Metaphysics of God’s Absoluteness (pp. 81-82). Pickwick Publications, An Imprint of Wipf and Stock Publishers. Kindle Edition.
But immutability of ethics can be its own consideration. God is consistent in character. Just because a human being has the ability to murder another human being, we do not require a metaphysical explanation as to why he doesn’t.